1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with a magnetic tape using a ferromagnetic metal powder, whose running property is improved without lowering S/N by the provision of a back layer.
2. Description of the Prior Art
As ferromagnetic powders, there are iron oxide type fine powders which have commonly been used, and ferromagnetic metal powders which have lately been used for the purpose of increasing the magnetic recording density as well as reproducing output due to their higher saturation magnetization and coercive force.
A magnetic recording tape using a ferromagnetic metal powder in a binder (so-called metal tape) is capable of giving a higher recording density than the iron oxide-type tape of the prior art and accordingly, its practical use as a video tape has lately been studied in real earnest. A trial device has been proposed in which VTR is subminiaturized by the use of such a metal tape and integrated with a camera. In this system the recording density is at least two times as much as that of the present VHS system or beta system and the diameter of a head cylinder and the head/tape relative speed are respectively reduced to about 1/2 of the present system. Accordingly, the recording wavelength is also reduced to half, i.e. at most 1 micron, approximately 0.6 micron.
Under these conditions, an image quality which compares favourably with the ordinary VHS/beta system is required for playback too. In assessment under the same conditions, therefore, a C/N ratio (playback signal output/modulation noise ratio in the case of recording FM carrier wave) of at least +6 is required for VHS/beta tapes. For satisfying these requirements, various conditions must be met and accordingly some problems arise.
That is, it is necessary to increase the coercive force (Hc) of a tape to some extent with shortening of recording wavelength. Moreover, it is desirable to hold the surface property as high as possible so as to reduce the spacing loss between a head and a base to the minimum. In order to improve the surface property of a magnetic layer, it is necessary to increase the dispersion of magnetic particles and to effect sufficient formation of the surface. This is effective for decreasing noise. Furthermore, it is more effective and important to decrease the particle size of magnetic particles. However, the decrease of particle size causes problems in that the surface electric resistance of a magnetic layer is increased and the potential of static charge of a tape during running in a VTR is raised, resulting in electric discharge and attraction of dusts being a cause of drop out. These problems tend to arise often under a low humidity and in particular, when a tape is fast forwarded or rewound.
As a non-magnetic support, there are commonly used plastic films such as polyethylene terephthalate films and polycarbonate films, but with the increase of recording density, a higher surface property is required on not only a magnetic layer side but also the back side thereof. If the surface property is upgraded, the friction coefficients with guide parts of a VTR, mechanical parts of a cassette and head cylinders are increased to lower the tape running property.
For the miniaturization of VTR, it has been necessary to make tapes thinner and accordingly, it has also been necessary to make thinner supports thereof. With the appearance of such thinner tapes, strength of the tape in the width direction is required such that breakage from the tape edge does not occur.